High growth rate of GaAs more than 90 mu m/h is achieved in the attempt of the productivity improvement of solar cells. The design concept is derived from the MOVPE reactor for nitrides. The design criteria for nitride MOVPE reactor were found to be very useful for GaAs MOVPE. Originally, MOVPE reactors for nitrides were developed on the basis of the reactor design for classical III-V materials. Today, the principle of reactor design to cope with the singularity of nitrides brings about essential feedback to the next-generation reactors for classical III-V materials for improved productivity. At first, design criteria for GaN MOVPE reactor are described in light of the vapor phase reaction. Secondly, cost analysis of a 19 mu m-thick GaN layer on a bulk GaN substrate for a vertical diode is briefly discussed in light of a trade-off between a high purity and a high growth rate of GaN. In GaAs-based solar cells, the thickness of light absorber is a couple of micrometers, accompanied by a thin (Al) InGaP cap, the growth time of which can be reduced to a few minutes if the growth rate is on the order of a few tens mu m/h. In consideration of these factors, cost analysis of MOVPE for GaAs solar cells will be discussed in comparison with GaN on GaN diode.